Pedicle screw fixation has become the favored mode of rigid internal fixation of the three vertebral columns of the spine. The use of pedicle screw-based instrumentation systems has steadily increased over the past three decades due to their superior biomechanical properties and higher bony fusion rates. However, violation of the pedicle medial wall or vertebral body while implanting these screws carries the risk of injury to neural, vascular, and visceral structures. For example, the screw body can break through the vertebral cortex causing direct trauma to the spinal cord or injury to nearby nerves (para-spinal nerves, nerve roots, etc) during positioning.
Pedicle screws are inserted into the spinal pedicle, the bony process projecting backward from the vertebral body, and stabilized with connecting rods or plates placed longitudinally with regard to the vertebrae. Biomechanical tests of pedicle-screw constructs have demonstrated the critical importance of screw placement in the isthmus of the pedicle to obtain proper cortical purchase necessary to resist screw pullout. Thus, surgical pedicle screw placement is technically demanding. Extensive training and meticulous attention to detail are required to avoid injuring the patient.
Despite the surgeon's skill, misplaced pedicle screws are common. Consequently, a number of computer-assisted surgical navigation systems and intraoperative fluoroscopy techniques have been developed to increase the accuracy rate of pedicle screw placement; however, these imaging systems have certain drawbacks. Fluoroscopic procedures increase radiation exposure to the patient, the operating room personnel, and the surgeon. Computer-assisted navigation systems require a pre-operative CT scan, exposing the patient to additional radiation and some surgeons perceive it as too time consuming and complicated to justify its routine use. Oftentimes, these imaging systems are limited in terms of resolution, that is, a breach of the medial wall of the pedicle may not be detectable.
While all of the aforementioned systems may enable a surgeon to determine if the screw has in fact breached the pedicle wall, this breach or crack is discovered only after the bone screw has been implanted. This requires the withdrawal and re-insertion of the screw after it may have already caused damage. Disruption of the pedicle wall by the installation equipment (K-wires, distractor systems, drill bit, awl, curette, etc) does not usually cause neural deficit. A minor crack or breach created by the installation equipment may not be detectable by an electronic test probe placed inside the pedicle screw opening. Most significant nerve damage occurs during the process of implanting the pedicle screw into the bony structure. During installation of the pedicle screw, a portion of the screw may inadvertently contact the nerve through the breach created by the installation equipment. This can give rise to neurological trauma, sensory deficit, or pain. Consequently, a need exists in the surgical arts for a system capable of providing proper fastener placement during implantation process to actively negotiate around or past nerves to prevent damage or improper screw placement.